Conventionally, there is known a wiring substrate having an insulating layer that is to be its an outermost layer and a pad exposed from the insulating layer. A connection terminal for connecting to a semiconductor chip or the like is formed on the pad. For example, a solder bump is used as the connection terminal.
The solder bump is formed on a pad by, for example, applying a paste-like solder (e.g., solder cream) on the pad exposed from an insulating layer and heating and performing a reflow process on the solder to heat and melt the solder. Thereby, a substrate having a solder bump formed thereon is obtained. According to necessity, the substrate is cleaned. Thereby, the manufacturing of a wiring substrate is completed.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2011-199179
However, during the process of applying the paste-like solder (e.g., solder cream) on the pad exposed from the insulating layer, a portion of the solder may adhere to an outermost surface of the insulating layer. It is to be noted that the term “outermost surface” refers to either one or both main surfaces of the wiring substrate positioned on the outermost side of the wiring substrate.
Although it is desirable for the portion of the solder adhered to the outermost surface of the insulating layer to flow on top of the pad during the reflow process, the portion of the solder may not flow on top of the pad in a case where the outermost surface of the insulating layer has good wettability (i.e., a case where the outermost surface of the insulating layer has a hydrophilic property). In this case, the portion of the solder may solidify and become residue on the outermost surface of the insulating layer.
The residual solder formed on the outermost surface of the insulating layer may detach from the outermost surface of the insulating layer during another process and cause short-circuiting with another electronic device, between pins such as connectors, or between wirings. This degrades the reliability of the wiring substrate.
Further, residue of flux contained in solder cream or the like may adhere to the outermost surface of the insulating layer. The residue of flux may create current leakage and cause insulation failure. Further, the residue of flux may be introduced to a contact of a connector or the like and cause connection failure. This degrades the reliability of the wiring substrate.